Majority Leader Harry Reid issued an emotional plea as he launched debate Monday on the most sweeping gun control legislation to reach the Senate floor in nearly two decades. But fundamental questions remain, including what the bill’s final language will be and whether Republicans can block the debate from proceeding.

The legislation (S 649) would require background checks on nearly all gun sales, impose tough new criminal penalties on firearms traffickers and authorize new funding for school security improvements.

In formally opening debate on the measure, Reid did not provide an update on the status of closed-door negotiations on the background check requirement, which has emerged as the White House’s top gun priority and is by far the most contentious aspect of the overall package.

Instead, the Nevada Democrat assailed a group of 13 Republican senators who sent him a letter threatening to block a motion to proceed to “any legislation that will serve as a vehicle for any additional gun restrictions.”

The group initially consisted of three first-term senators with tea party backing — Rand Paul of Kentucky, Ted Cruz of Texas and Mike Lee of Utah — but on Monday expanded dramatically. It now includes establishment Republicans such as Marco Rubio of Florida, who is widely considered a potential presidential candidate in 2016.

Later Monday, Minority Leader Mitch McConnell, R-Ky., also joined the group’s call for a filibuster on the motion to proceed, at least on the legislation as it is currently written. McConnell’s announcement threatened to sharply escalate the procedural fight over the bill.

“The least Republicans owe the parents of these 20 little babies who were murdered at Sandy Hook is a thoughtful debate about whether stronger laws could have saved their little girls and boys,” Reid said on the Senate floor, referring the shooting massacre at an elementary school in Newtown, Conn., in December. “The least Republicans owe them is a vote.”

Reid also accused Republicans of hypocrisy.

Republicans “call for free and open debate in the Senate, and those yelling the most for this free and open debate are the people who sent me a letter saying, ‘We’re going to filibuster everything that relates to guns,’” he said. “Talk about speaking out of both sides of their mouths.”

Background Check Talks

While the group of 13 GOP senators appeared inclined to block debate on any bill with new gun restrictions, McConnell did not rule out allowing debate on the legislation to proceed if the measure is changed. That is a possibility, given ongoing negotiations over the background check component of the package.

A bipartisan group of senators — including Democrats Charles E. Schumer of New York and Joe Manchin III of West Virginia, as well as Republicans Mark S. Kirk of Illinois and Tom Coburn of Oklahoma — has tried for months to find common ground on the background check plan. But the language currently in the bill, drafted by Schumer and preferred by the White House, has no GOP supporters and could derail the entire legislation unless it is modified.

Schumer’s language would ensure that records are kept of all gun sales, including those between private individuals. Record keeping has been a nonstarter for Coburn and other conservative Republicans, who fear it will lead to a national registry of gun owners.

Reid has said he is open to swapping out the Schumer language with an alternative that stands a better chance of attracting bipartisan support, but no such alternative has emerged.

Manchin and Coburn are developing their own proposals that would stop short of Schumer’s background check proposal, and Pennsylvania Sen. Patrick J. Toomey has emerged as a late addition to the group of negotiators. Toomey, a conservative Republican with a top rating from the National Rifle Association, could be crucial to securing a deal that would be amenable to both sides.

On Monday, President Barack Obama cranked up the pressure on the Senate by traveling to Connecticut to urge lawmakers to find a compromise on the background check plan. Absent such a deal, the White House would have difficulty claiming any significant political or policy victory despite months of trying.

“If you’re an American who wants to do something to prevent more families from knowing the immeasurable anguish that these families know, now is the time to act,” Obama said. “Now is the time to make your voice heard from every statehouse to the corridors of Congress.”

ACLU Concerns

While Democrats have worked hard in recent weeks to dispel fears among Republicans and gun rights groups that the background check plan would lead to a national registry of gun owners, the American Civil Liberties Union now is voicing just that concern. The group’s objection has presented yet another complication for Democrats who are pushing the bill.

Chris Calabrese, the ACLU’s legislative counsel for privacy-related issues, said in an interview with CQ Roll Call that the Schumer background check language, as it is currently written, could present serious privacy concerns because it would allow the government to retain records of private gun sales that are not currently retained for commercial gun sales. Calabrese, echoing an argument of the NRA, characterized the proposal as a slippery slope to a registry.

“Records get created for one purpose, and they’re sitting there and somebody says, ‘Boy, it would be really great to use that for something else,’” Calabrese said. “We’re not saying the Schumer legislation does that, but that’s frequently what happens when you mandate the keeping of records.”

The ACLU also has raised privacy concerns about other aspects of the legislation, including school security proposals that could lead to more surveillance equipment and tip lines to report potentially dangerous students.

Calabrese declined to say whether his office has been in touch with Sen. Ron Wyden, D-Ore., and other leading privacy advocates in the Senate over details of the gun legislation that have been flagged by the ACLU. A spokesman for Wyden did not immediately respond to a request for comment.

But gun rights groups, such as the Citizens Committee for the Right to Keep and Bear Arms, are strongly encouraged by the ACLU’s position, arguing that it validates their long-standing concerns about the gun legislation.